Replace some object comparisons with type checks

`JsonNull` is a non-`data` object, meaning its `==` is not safe for comparing
instances of `JsonNull` after deserialization. After ^KT-76635, such comparisons
are discouraged in favor of `is`-checks.

I presume turning `JsonNull` into a `data` object may be a decision requiring
additional attention, so I don't want it to block the MR related to ^KT-76635.
Intuitively, it would probably be safe if we also add a custom `toString()`
override that would match the default one?

The same story with the checks inside `selectMode()`.

Author: lunakoly

formats/json/commonMain/src/kotlinx/serialization/json/internal/TreeJsonDecoder.kt

@@ -20,7 +20,7 @@ public fun <T> readJson(json: Json, element: JsonElement, deserializer: Deserial
     val input = when (element) {
         is JsonObject -> JsonTreeDecoder(json, element)
         is JsonArray -> JsonTreeListDecoder(json, element)
-        is JsonLiteral, JsonNull -> JsonPrimitiveDecoder(json, element as JsonPrimitive)
+        is JsonLiteral, is JsonNull -> JsonPrimitiveDecoder(json, element)
     }
     return input.decodeSerializableValue(deserializer)
 }

formats/json/jsMain/src/kotlinx/serialization/json/internal/DynamicEncoders.kt

@@ -235,10 +235,10 @@ private class DynamicObjectEncoder(
 
     @OptIn(ExperimentalSerializationApi::class)
     fun selectMode(desc: SerialDescriptor) = when (desc.kind) {
-        StructureKind.CLASS, StructureKind.OBJECT, SerialKind.CONTEXTUAL -> WriteMode.OBJ
-        StructureKind.LIST, is PolymorphicKind -> WriteMode.LIST
-        StructureKind.MAP -> WriteMode.MAP
-        is PrimitiveKind, SerialKind.ENUM -> {
+        is StructureKind.CLASS, is StructureKind.OBJECT, is SerialKind.CONTEXTUAL -> WriteMode.OBJ
+        is StructureKind.LIST, is PolymorphicKind -> WriteMode.LIST
+        is StructureKind.MAP -> WriteMode.MAP
+        is PrimitiveKind, is SerialKind.ENUM -> {
             // the two cases are handled in DynamicObjectSerializer. But compiler does not know
             error("DynamicObjectSerializer does not support serialization of singular primitive values or enum types.")
         }